Houdini 21.0 Unreal

Materials

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You can assign materials to meshes generated by the plug-in in two different ways:

Houdini Engine can also automatically create material instances from an existing Unreal material and procedurally change its parameters.

By Attribute

The special Unreal to Houdini primitive attribute unreal_material specifies an Unreal material asset to apply to a given primitive. This attribute has the path to the asset as its string value. You can get the path to the material asset by right-clicking and choosing Copy reference in the context menu.

If the sent mesh has more than one material, the value of the unreal_material attribute will be prefixed by its material slot number in square brackets, for example:

[0]/Script/Engine.Material'/Game/StarterContent/Materials/M_Ground_Grass.M_Ground_Grass

You can also automatically create a Material Instance of a given Unreal material asset. See Material Instances for more information. You can change a physical material by using the unreal_physical_material attribute.

Generate

Optionally, the plug-in can attempt to generate Unreal materials based on the Houdini materials found on the asset.

The plug-in recognizes three ways in Houdini to assign a material:

  1. Using a Material network and assigning it to the geometry using a Material node.

  2. Using a SHOP network and the shop_materialpath attribute.

  3. Using a COP2 or Copernicus (COP) network and assigning it to the geometry using a COP Preview Material node.

The material reads each primitive/face in the produced geometry and creates an Unreal material for each unique Houdini material.

To generate materials from your HDA using a Principled Shader:

  1. Ensure the mesh generated in your HDA has UVs.

  2. Create a Material network in your HDA.

  3. Create a Principled Shader inside the Material network.

  4. At the GEO/SOP level, add a Material node to your generated geometry. Have the Material node’s Material parameter point to the Principled Shader in the Material network.

To generate materials from your HDA using a COP Preview Node:

  1. Ensure the mesh generated in your HDA has UVs.

  2. At the GEO/SOP level, add a COP Preview Node to your generated geometry.

Only some of the material channels will be translated to the Unreal material. See the below table for which ones.

On a SHOP node, Principled shader, or COP preview node, there are multiple ways to assign a material channel. For example, the base color channel has both a texture parameter and a constant color parameter. The plug-in chooses only one parameter to use for the material, and prefers textures over constant values.

The plug-in first searches for parameters by name. If a parameter of that name cannot be found, it searches for a parameter with a matching compatibility tag.

The names and tags are:

Unreal Material Channel

Parameter

Compatibility Tag

COP Preview Node Parameter

Base Color

basecolor

ogl_diff

constantbasecolor

default_basecolor

Base Color (Texture)

basecolor_texture

ogl_tex1

basecolor

Metallic

metallic

ogl_metallic

constantmetalness

default_metalness

Metallic (Texture)

metallic_texture

ogl_metallicmap

metalness

Specular

reflect

ogl_spec

constantspecularamount

default_specular

Specular (Texture)

reflect_texture

ogl_specmap

specular

Roughness

rough

ogl_rough

constantspecularroughness

default_specular_roughness

Roughness (Texture)

rough_texture

ogl_roughmap

roughness

Emissive Color

emitcolor

ogl_emit

constantemissioncolor

default_emission_color

Emissive Color (Texture)

emitcolor_texture

ogl_emissionmap

emission_color

Emissive Intensity

emitint

ogl_emit_intensity

constantemissionamount

default_emission_amount

Opacity

opac

ogl_alpha

constantopacity

default_opacity_amount

Opacity Mask (Texture)

opaccolor_texture

ogl_opacitymap

opacity

Normal (Texture)

baseNormal_texture

ogl_normalmap

normal_map

Normal Map Type (Tangent/World)

ogl_normalmap_type

Note

ogl_normalmap_type determines the space of the normal map (Tangent vs World). If the parameter is set to World Space, then the plug-in sets the material’s bTangentSpaceNormal UProperty to false and the normal map will be in World space. If not, the normal map will be in Tangent space.

Standalone Texture Import

If you are using NodeSync, you can directly fetch a COP2 or Copernicus (COP) texture from Houdini, without having to attach it to a material. It will be translated into an Unreal texture. See the NodeSync page for more information on using NodeSync.

Material Instances

You can automatically create a Material Instance from a given Unreal Material asset.

  1. Assign the unreal_material_instance primitive attribute

  2. For the parameter’s string value, use the path to the material asset.

  3. To obtain the path, right-click on the asset and choose Copy reference.

Upon cook, the plug-in creates a new Material instance of the source Unreal Material in the cook temp folder and assigns it to the generated geometry.

For landscapes, use unreal_material_instance for the Landscape Material, and unreal_material_hole_instance for the hole material.

Material Instance Parameters

You can use material instance to control and modify material parameters in the attributes.

Change the material parameters in the generic unreal_material_parameter_ prefix. This works similarly to Generic UProperty Attributes

  1. Add a detail or primitive attribute that begins with unreal_material_parameter_

  2. Append the name of your material parameter without spaces.

By default, the plug-in applies this override to any materials that have a parameter matching this name. If desired, you can specify the index of the material slot to apply this override to by prepending the material parameter name with INDEX_ where INDEX is the material slot index. This only works if the unreal_material_instance values have their corresponding material index prepended to the values. Otherwise, the index of each material is set according to whichever materials appear first in the list of unreal_material_instance values.

This system works both for the standard material parameters that are always generated by Unreal when creating a material instance (BlendMode, PhysMaterial, TwoSided) but also for the custom material parameters that expose scalar, vector or texture parameters defined in your material.

When it cooks, it overrides and sets its value to your specifications. This works for both the standard material parameters generated by Unreal and custom material parameters that expose scalar, vector, or textureparameters.

For example:

  • To set the Diffuse Boost parameter to 2.0, add a unreal_material_parameter_diffuseboost detail attribute to your asset, and set its value to 2.0 (Float attribute)

  • To change the Blend Mode parameter from the default Opaque value, to Additive, add a unreal_material_parameter_blendmode detail attribute to your asset, and set its value to either additive (String attribute) or 3. (Float or Integer attribute)

  • To change a custom My Color Vector parameter that you created in your material, add a unreal_material_parameter_mycolor detail attribute, Float with a tuple size of 4, and set its value to the desired color value, ( 1.0, 0.0, 0.0, 1.0 ) for a bright red

This system currently supports 4 different types of material parameters:

  • Scalar parameters, which you can control with single tuple attributes (Float or Integer).

  • Vector parameters, which you can control with Float or Integer attributes with a tuple size of 4 or 3. Unreal will interpret Float attributes as an FLinearColor, and Integer attributes as an FColor.

  • Enum parameters (like Blend Mode), which you can set either by using the string value of the enum or the integer value of int (starting from zero).

  • Texture parameters, which you can change with String attributes. To use an existing unreal texture asset, you can simply use the asset’s unreal path (obtained by right clicking on it and choosing Copy Reference).

To use a texture generated by the current Houdini Asset, you have two options:

  1. For a single material, set the attributes to the material channel (basecolor (or diffuse), normal, specular, roughness, emissive, metallic, opacitymask).

  2. For multiple materials, add the relative path to the SHOP node(or principled shader node) that generates the material before the desired channel. For example testgeometrysquab1/shopnet1/SquabSkin/diffuse. This path is relative to your asset node.

Attributes

Houdini to Unreal

This table lists special attributes to set in an HDA and the plugin recognizes when you translate the Houdini data to Unreal outputs. You can setup these attributes on your assets to control and override some properties and behaviours of the generated Unreal outputs. See Attributes and Groups for more information.

Materials

Attribute Name

Owner

Type

Description

unreal_material

any

string

Unreal path of a Material asset to apply to an output. You can specify the material slot by prefixing the path with the slot number in square brackets.

unreal_physical_material

prim, detail

string

Unreal path of a PhysicalMaterial asset to be used as a Physcial Material on the output

unreal_simple_physical_material

prim, detail

string

Unreal path of a PhysicalMaterial asset to be used for Simple Unreal Collisions

unreal_face_smoothing_mask

prim

int

Mesh smoothing mask data

Material Instances

Attribute Name

Owner

Type

Description

unreal_material_instance

prim, detail

string

Unreal path of a Material asset to create an instance of

unreal_material_hole_instance

prim, detail

string

Unreal path of a Material asset to create an instance of, to be used for Landscape holes

unreal_material_parameter_*

detail

any

Attribute prefix used to modify generated material instances' parameters

Unreal to Houdini

This table lists special attributes which are created by the plug-in when importing data from Unreal to Houdini. See Attributes and Groups for more information.

Materials

Attribute Name

Owner

Type

Description

unreal_simple_physical_material

prim

string

Unreal path of a PhysicalMaterial asset taken from simple collisions

Unreal

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